Multiple site communication systems are known to comprise a plurality of sites, a plurality of communication units, such as mobile and portable radios, and a limited number of communication resources which are allocated to the communication units by at least one communication resource allocator, such as a central controller. Generally, a communication between two or more communication units is established when a communication resource is allocated for the communication units to use. If the communication units are located within the same site, only one communication resource needs to be allocated for the communication. If, however, the communication units are located in different sites, then a communication resource needs to be allocated in each site for the communication to occur.
For a group call (a call among several communication units that are grouped together based on commonalty of use), a communication resource must be allocated in each site that supports a member of the communication group, i.e., a member is located within the site. With such a requirement, group calls tend to take a noticeable amount of time (1 second to 30 seconds) to set-up. Most of the set-up time is spent waiting for communication resources to become available in sites that service members of the communication group. Once all the sites have an available communication resource, the group call can be executed. As would be expected, the larger the group, the longer it takes to place the group call.
There are several effective solutions for reducing the set-up time of a group call. One such solution is generally known as "Busy Override". In a busy override system, the requesting unit can elect to begin a group call without the members of the communication group that are located in sites that do not have an available communication resource. For the busy override system to be effective, the group members located in the sites that do not have an available communication resource cannot be essential participants to the group call. Another solution targets particular members of the communication group that are essential to the group call and communication resources are only allocated in the sites in which the essential members are located. Other members located in these sites can also participate in the group call, even though they were not identified as essential participants.
While both of these solutions work well to minimize set-up times for group calls, they provide little benefit for subsequent group calls that are initiated by the same communication group shortly after a group call has ended. In these situations, the subsequent call is treated like any other call and a complete set-up routine needs to be executed. Note, however, that statistical user information indicates that if a subsequent group call is initiated by the same communication group within a certain time (say, less than 10 seconds) after a group call has ended, there is a high probability that the topic of the subsequent group call will be related to that of the previous group call. For example, if the first group call was established by a communication group operated by a fire department in response to a fire, a group call initiated a few seconds after the first group call ended would still probably be related to the fire. As such, the members that participated in the first group call would more than likely be the desired members to participate in the second group call.
Therefore, a need exists for a method of maintaining audience continuity for subsequent group call requests for a previously active communication group without having to invoke an entire group call set-up routine.